Acid-fast staining is used to detect which organisms?

• A. Mycobacteria, including M. tuberculosis and M. leprae
• B. Gram-positive cocci
• C. Fungi
• D. Gram-negative rods

Answer: (B)

Acid-fast staining targets organisms with a waxy, lipid-rich cell wall that contains mycolic acids. This thick, lipid layer resists decolorization by acid alcohol, so after staining, these bacteria retain the primary dye and appear bright red against a blue or green counterstain. In practice, Ziehl-Neelsen or similar methods are used. The classic organisms detected are Mycobacterium species, including the tuberculosis-causing M. tuberculosis complex and the leprosy-causing M. leprae. Some other organisms, like Nocardia, can be weakly acid-fast, but the primary clinical use is for Mycobacteria.

In contrast, Gram-positive cocci, Gram-negative rods, and fungi do not have that waxy mycolic-acid–rich cell wall, so they lose the primary stain during acid-alcohol decolorization and do not appear acid-fast. They are identified by other staining techniques specific to their cell wall properties or by fungal-specific stains.